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Two friends at the office raved about their home exercise rooms for more than a year. They get up early, do a full workout three times a week before leaving for work, and they always look buff. It was an idea whose time had come, especially since you bought a new house with a spare bedroom in a place that’s miles from the gym where you regularly worked out.

This great idea, however, did not turn out the way you anticipated. The first time you used your treadmill, your husband came running up the stairs yelling, “When you run on the treadmill the dishes in the wall cabinets in the kitchen are clattering and driving me crazy! Even the baby is crying.”

Monday morning quarterbacking this disaster, what could you have done differently? And what, exactly is the problem?

With home exercise rooms, two things can upset homeowners–shaking and noise, said Dan Dolan, a professor of structural engineering at Washington State University in Pullman, Wash. Even more confounding, homeowners often confuse the two. Treadmill pounding can cause shaking, but when you’re underneath it, the discomfort is usually due to noise. When you’re on the same floor as the exerciser, you are more likely to feel the shaking and be annoyed by it.

A number of things account for the shaking, Dolan said.

First of all, wood, the material used to frame the house, is flexible. When weight is put on wood floor joists, the structural members that support floors in wood-frame houses, the wood can deflect or sag slightly and then bounce back. This is the phenomenon that people feel when they walk across a floor and pronounce it “bouncy.”

The bouncy floor phenomenon is relatively recent and more or less parallels the home-buying public’s penchant for bigger houses with bigger rooms and open floor plans with fewer walls, Dolan said. To provide this, the builders needed a different type of floor joist that could span longer distances. Instead of the solid-sawn “dimensional” lumber joists that come straight from the tree, many builders now use a floor joist that looks like a steel I-beam that is made out of wood. Not surprisingly, it’s called a wood I-joist.

With longer spans, the wood I-joist can deflect more. And because the I-joist is lighter, it doesn’t take much of a load to produce a bouncing sensation on the longer spans. A small child or a 50-pound dog running across a floor can cause it.

Some floors can deflect quite a bit. Dolan said in one instance he observed the legs of a coffee table bounce up three-quarters of an inch when he did a footfall on a floor with joists that spanned 24 feet. Though alarming, the structure was perfectly safe.

The deflection created when someone is pounding on a treadmill is far less dramatic, but still annoying. Even smaller floor deflections, which you would sense as vibrations, can also be annoying, Dolan said. When a floor joist vibrates in the range of 4 to 12 cycles per second, you will feel it. When the vibrations are 7 cycles per second–the same frequency as many of the organs in your body including your brain–most people will be annoyed, even if the vibrations are not strong.

The degree of annoyance, however, varies with the individual. Some people can be very distressed, while others can be nearly oblivious, Dolan said.

If you think the floor deflections and vibrations from a second-floor exercise room might bother you, Dolan suggested the same footfall test he used in the living room with the bouncing coffee table–rise up on your toes and then let your heels fall to the floor.

Perform the test in the builder’s model or a finished house that most closely resembles the floor plan you are considering because small differences in room sizes and plan configurations can make a big difference in your perception of floor deflections and vibrations, Dolan said.

The exercising partner should walk around the proposed exercise room making a footfall test every few steps. The other person should stand in the bedroom across the hall and in the adjacent bedroom, with the door shut to separate out the noise from the physical phenomenon. If the footfall-induced vibrations are objectionable, you may be unhappy, second-floor exercise room or not. If the footfall test went well, have the exerciser mimic the treadmill motions, while the other person stands in the neighboring bedrooms.

Next, assess the effects of the simulated treadmill jogging on the first floor. Downstairs you may hear the pounding and observe chandeliers or hanging fixtures swaying. You won’t feel the vibration or deflection, but it can cause dishes kept in wall cabinets to clatter, so check their location. If the cabinets are attached to the wall that supports the exercise room or are directly under it (some open plans suspend the cabinets from the ceiling in the middle of a room) stacked plates kept in it will clatter.

If your assessment finds the house wanting, what are your options? Because a production builder is unlikely to modify his framing to suit one buyer, your only recourse is to keep looking at other models or put the exercise equipment in the basement.

But, if you are building a made-for-you custom house and you want the second-floor exercise room to get the inspiring view, you can instruct your architect or builder to make the floor stiffer, said James Madison Cutts, a structural engineer in Washington, D.C. Even if exercise is not the issue, some clients want a stiffer floor because the wife says, “when I walk across the floor I don’t want to see my jewelry bounce,” he added.

Re-engineering the floor will not eliminate the bounciness and vibrations entirely, but it can make a dramatic difference, Cutts said. First, add depth to the floor joists. Instead of having ones with a 10-inch depth, get ones with a 12 or 14-inch depth. Next, add an extra layer of ¾-inch subflooring and glue and screw both to the floor joists. Increasing the mass and weight of the floor structure will also reduce its movement, and Cutts would put concrete between the joists. These cures sound simple enough, but Cutts stressed that they are so unusual for residential construction you need to consult a structural engineer.

Then there’s the noise issue. Chris Savereid, an acoustics consultant with Acentech in Cambridge, Mass., said that unlike the sound waves generated by a television or a CD player, which are airborne, the noise generated by someone using exercise equipment is transmitted through the structure itself. To reduce this, you need to devise a strategy that decouples the second-floor structure from the ceiling in the room below.

For example, Savereid suggested creating a separate supportive frame for the ceiling and hang it from the side walls of that room. If that room is very large, however, this is not practical. Another possibility is attaching the ceiling to resilient hangers, which, in turn, are attached to the floor joist.

But, Savereid explained, these solutions will not completely eliminate the sound of the pounding treadmill because it can still travel through other parts of the rigid frame, which connects the second-floor to the first.

An easier fix is to locate the exercise room over a garage if your new house will have one, or in a basement. But before you set up in the basement, which many people find dank and dark, ask other family members about the noise, Savereid advised. In many cases, people will tolerate noise from their loved ones that would be unacceptable from anyone else.

Just ask Jim Cutts. “We have a treadmill and my wife jogs on it all the time. If you’re in my house you can feel it, but it doesn’t bother me at all.”

Queries or questions? Katherine Salant can be contacted at www.katherinesalant.com.

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